1. Field of the Invention
The present invention relates to a semiconductor device and a method of fabricating the same, and more particularly, to a semiconductor device having a decoupling capacitor and a method of fabricating the same.
2. Description of Related Art
A decoupling capacitor of a semiconductor device is required by most very large scale integration (VLSI) circuits in order to minimize a noise effect. The decoupling capacitor is connected to various power supplies of logic and memory array circuits. Irrespective of whether power is externally supplied or internally produced, there is typically a variation of about −0.5 to 3.5 V in the power supplied on an advanced DRAM array. As a result, it is difficult to provide a decoupling capacitor adequate for all various power supplies.
An external power supply Vext of about 1.8 V is internally boosted or dropped to generate internal power supplies. In this case, a boosted internal power supply Vpp is internally generated by a charge pump and regulated by a voltage regulator. While data is being read from a DRAM cell or written in the DRAM cell, a word line accesses the boosted internal power supply Vpp. The boosted internal power supply Vpp is normally much higher than the external power supply Vext. As an example, when DRAM technology provides an external power supply Vext of 1.8 V, the boost internal power supply Vpp may be 3.0 V. The boost internal power supply Vpp is needed to increase the access time of the DRAM. Thus, when the word line accesses the boosted internal power supply Vpp, a large current is supplied from the boosted internal power supply Vpp. If a decoupling capacitor of the boosted internal power supply Vpp does not have sufficient capacitance, a noise spike may occur in the boosted internal power supply Vpp.
Also, the external power supply Vext may be dropped to produce a reduced internal power supply Vint. The dropped internal power supply Vint serves to supply power to portions of the semiconductor device other than the word line, which need low power.
In general, a decoupling capacitor of a conventional semiconductor device is filled in an empty space of a peripheral circuit region outside of a cell region. Accordingly, a region filled with the decoupling capacitor is restricted, and it is necessary to maximize the capacitance of the decoupling capacitor in the restricted region to enhance a filtering effect.
A method of forming a decoupling capacitor having a large capacitance in the restricted region is disclosed in Korean Patent Registration No. 329,616 entitled “Method of Forming Decoupling Capacitor of Semiconductor Device.”
According to Korean Patent Registration No. 329,616, the method includes forming a plurality of trenches in a semiconductor substrate having a decoupling capacitor forming region, forming a gate oxide layer and a conductive layer for a gate electrode on a surface of the semiconductor substrate, and forming a gate electrode and a decoupling capacitor by patterning the gate oxide layer and the conductive layer. This method employs the trenches in order to increase the capacitance of the decoupling capacitor. However, when the decoupling capacitor is formed between power lines (e.g., a boosted internal power supply Vpp line and a ground GND line) having a high electrical potential difference, there is a stronger likelihood that a breakdown phenomenon will occur because of the gate oxide layer with a small thickness. Alternatively, when the gate oxide layer is formed to a large thickness to prevent the breakdown phenomenon, a decoupling capacitor formed between power lines with a low electrical potential difference has a reduced capacitance due to the thick gate oxide layer.
In another approach, a method of varying a decoupling capacitor with respect to a high voltage and a low voltage is disclosed in Korean Patent Laid-open Publication No. 2002-58019 entitled “Optimized Decoupling Capacitor using Lithographic Dummy Filler”.
According to Korean Patent Laid-open Publication No. 2002-58019, a deep trench lithographic fill pattern may be used to form a decoupling capacitor for a low-voltage power supply including an internal low voltage for inputting and outputting data, a bit line equalization voltage, a bit line high voltage, or a negative word line voltage. For a high-voltage power supply, a plate capacitor or a field effect transistor (FET) capacitor may be formed by combining a gate filler with a diffusion filler. The formation of the plate capacitor or the FET capacitor may include forming a thin gate oxide capacitor for an external supply voltage or forming a thick gate oxide capacitor for a boosted word line supply voltage. The above-described method proposes a method of varying the thickness of the gate oxide layer with respect to a high-voltage power supply but does not provide a solution to a reduction in the capacitance of the decoupling capacitor caused by an increase in the thickness of the gate oxide layer.